This invention relates to a dip roll for applying a coating to a web of paper and a method of making the dip roll, and more particularly, to a dip roll having a plurality of cupped indentations on its surface for increasing the transfer of the coating liquid from a supply to the dip roll and web, and a method of making such roll.
In the manufacture of coated paper, a coater having a dip roll running in a trough is sometimes used to transfer coating liquid from the trough to the web of moving paper. In such coater the dip roll is normally located adjacent a backing roll, with the web of paper being run between the nip of the dip roll and backing roll. The dip roll does not actually contact the backing roll, as usually a small gap is provided between the rolls to control the amount of coating to be transferred to the web.
Previous dip rolls were usually made of metal, but covered with a smooth surface of rubber or plastic material or compound. One of the functions of the covering was to enhance the transfer of the coating liquid carried by the dip roll from the trough, over that of a plain metal roll. However, the rubber or plastic covering has disadvantages in that it may swell with increases in temperature, thus changing or eliminating the desired gap between dip roll and backing roll. The changing of this gap has an adverse effect, as it alters the amount of coating applied to the web. Changes in the gap, due to roll swelling can also cause coating skips, increase undesired penetration of the coating into the web, and can increase the hydrodynamic force at the backing-dip roll nip. The covering also hardens and glazes with age so that after a period of time, it must be repaired or replaced. Unfortunately, the techniques for repairing such covered dip rolls, usually by regrinding the surface of the roll or recovering it, are slow, expensive and require special, costly equipment, not usually available at a paper mill. Further, the relatively soft covering is easily damaged, making for frequent need for repair or replacement.